The sphingolipid storage diseases are a group of ~40 genetically distinct disorders that result from inherited deficiencies of lysosomal hydrolytic activities or lipid transport. Among this group is Niemann-Pick C disease, an autosomal recessive disorder for which there is no effective treatment. Niemann-Pick C patients exhibit a clinically heterogeneous phenotype characterized by severe, progressive neurological impairment that is usually fatal in childhood. Most cases are caused by loss-of-function mutations in the NPC1 gene, resulting in impaired intracellular trafficking of cholesterol and glycosphingolipids. Despite increased knowledge of how NPC1 facilitates intracellular lipid transport, it remains poorly understood how mutations in this gene lead to the severe neuropathology by which this disorder is characterized. This lack of knowledge hinders the identification of specific targets for developing disease-modifying therapies. The objective of this application is to identify mechanisms leading to neurodegeneration and to define cellular pathways where interventions could result in effective treatments. Our central hypothesis is that neurological disease is governed by Npc1 deficiency in neurons by triggering neuron loss and impairing protein quality control. This hypothesis springs from our analysis of mice we generated with a conditional null allele of the Npc1 gene, thereby creating a novel tool for studying disease pathogenesis. Our preliminary studies show that Npc1 deletion in Purkinje cells is sufficient to trigger neuron loss in anterior, but not posterior cerebellar lobules. Our findings demonstrate that these subpopulations of neurons exhibit strikingly different vulnerability to the toxicity of Npc1 deficiency, and we hypothesize that this reflects underlying differences in gene expression. Further, we demonstrate that Npc1 deficiency impairs lysosomal protein quality control and hypothesize that defects in proteostasis underlie neuron loss. Behavioral, histological, biochemical and genetic approaches will be used to establish the extent to which Npc1 deficiency in neurons is sufficient to cause neurological disease (Aim 1), identify genes that protect neurons from the toxicity of Npc1 deficiency (Aim 2), and characterize the contribution of impaired proteostasis in Niemann-Pick C neurodegeneration (Aim 3). These studies are expected to have an important positive impact by defining pathways through which Npc1 deficiency leads to progressive neurodegeneration and by identifying potential therapeutic targets. Further, we expect that shared mechanisms mediate toxicity in several sphingolipid storage diseases, suggesting that advances here will impact our understanding and treatment approaches to genetically distinct lysosomal storage disorders.